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Biogas production from the aquatic weed Pistia (Pistia stratiotes)
Bioresource Technology 37 (1991) 211-214
Biogas Production from the Aquatic Weed Pistia
( Pistia stratiotes ) S. A. Abbasi, P. C. Nipaney Salim Ali School of Ecology, Pondicherry (Central) University, Pondicherry 605 001, India
& M. B. Panholzer Institute for Environmental Research, Elisabethstrasse 11, A-8010 Graz, Austria (Received 8 April 1990; revised version received 20 October 1990; accepted 30 October 1990)
Abstract Pistia stratiotes, an aquatic weed, was investigated as a substrate for biogas production in batch digestion. An inoculum was necessary to obtain biogas production from the weed. With Pistia only, production of carbon dioxide alone was high during the first five days of digestion but began to level off thereafter With inoculated Pistia, a high rate of biogas production was sustained for nearly 10 days and the average methane content was 58-68%. The digesters charged with Pistia alone had significant concentrations of propionic, butyric, isobutyric, valeric, and isovaleric acids. These acids were not present in detectable concentrations, in the digesters running with inoculated Pistia, except during the first 4 days of the digestion when propionic acid was formed. When an inoculum was added to a 'soured' digester the performance of the latter improved dramatically.
Key words: Aquatic weed, anaerobic digestion, biogas, batch digestion, Pistia, volatile and fatty acids, methane. INTRODUCTION
Pistia stratiotes L., commonly known as water lettuce or water cabbage, is a pleustonic macrophyte, originally from the states of Florida and Texas, USA (Bua-Ngam & Mercado, 1975; Attionu, 1976; Rao & Reddy, 1984; Tadulingam
& Venkatanarayana, 1985). It is now widespread in the lakes and ponds of the warmer parts of the world. Pistia has a high growth-rate and causes harm to water resources and agriculture in the same manner as other major aquatic weeds. It might be profitably used as a substrate for biogas production.
METHODS Sampling of P/st/a Fresh Pistia (whole) plants were collected from the Botanical Garden, University of Graz, Austria. The plants were washed with water to remove attached sediments, and then finely ground in an 'Ultra Turrax' homogenizer (Janke & Knkel, Staufen, Germany). Source of inoculum Digested slurry from a biogas plant (Harrer Biogas Plant, Graz) running with cow manure at 35°C for several years, was used as an inoculum for the present study. Chemical analysis of P/st/a and inoculum Total solids (TS), volatile solids (VS), nitrogen (N) and pH were determined as the standard methods (Rand et al., 1985). Ammonium nitrogen and chemical oxygen demand (COD) analyses were carried out according to the German Standard Methods of the examination of water, wastewater and sludge (Frachgruppe Wasserchemie, 1985).
211 Bioresource Technology 0960-8524/91/S03.50 © 1991 Elsevier Science Publishers Ltd, England. Primed in Great Britain
S.A. Abbasi, P. C. Nipaney, M. B. Panholzer
212
Volatile fatty acid (VFA) and methane analysis Volatile fatty acids and methane were estimated using a Hewlett Packard gas-liquid chromatograph.
Anaerobic digestion of Pistia All-glass air-tight digesters of 3000 ml capacity with gas sampling and measuring units were employed. Each digester set consisted of a 3000ml round-bottom flask as digester, calibrated gas collecting tube and a bottle to receive displaced water. The digesters had two outlet connections; one for gas and one to collect the slurry for analysis. The gas-collecting tube was filled with acidified water (pH N 2) to prevent absorption of carbon dioxide and hydrogen sulfide gases from the biogas. The digesters were charged with Pistia or Pistia-inoculum mixtures. The controls were with inocula alone.
RESULTS AND DISCUSSION Some characteristics of the feed are presented in Table 1.
Biogas yield The biogas yield per kg (fresh weight) of the feeds over a 30-day retention time is presented in Table 2. The methane content of the biogas from digesters A and C (inoculum/inoculated Pistia) was in the range 52-54% during the first 2 days of the digestion and remained in the range 61-68% for the remaining period. The biogas from digester B (Pistia without inoculum) consisted only of carbon dioxide. As might be expected the presence of inocula has a very pronounced positive effect on the biogas production from Pistia. The pattern of biogas yield -- HRT curves (not shown here) for Pistia was similar to the patterns Table 1. Some characteristics of the feed (wet weight basis)
Parameter
lnocula
Total solids (TS%) Volatile solids (VS%) Chemical oxygen demand (g O2 kg -1) Ammonium nitrogen ( g N k g -~) Kjeldahl nitrogen ( g N k g -~)
6.9 5.4 67
Pistia 4.6 3'9 36
Mixture 5.6 4-4 62
2.2
0.05
1"9
3-7
0-6
3'2
observed with Ceratopteris but different from the ones observed with Salvinia, Azolla, Hydrilla, Nymphaea, Cyperus, Scirpus and Utricularia (Abbasi & Nipaney, 1990). The nature of VS in Pistia is such that the bulk of degradation takes place during the initial phase of the digestion, suggesting that the digesters with Pistia should preferably be run at an HRT close to 10 days for optimal energy yield.
pH The pH of the digesters running on inocula or Pistia-inoculum mixtures was in the mildly alkaline range 7.4-8.0 whereas the digesters with "Pistia alone remained significantly acidic; the pH never rising above 5"3 and often remaining lower than 5 (Tables 3 and 4).
VFA Composition The nature and composition of volatile fatty acids formed during the course of the digestions are shown in Tables 3 and 4. It may be seen that in the case of Pistia, the acetic acid concentrations shoot up to 517 ppm by the 4th day of the digestion and remain in the range 563-978 ppm thereafter, indicating that while the plant is contaminated with fermentative bacteria, methanogenic bacteria are absent from Pistia plants. In contrast inoculated Pistia has a low concentration ( < 70 ppm) of residual acetic acid from the 8th day onwards.
Table 2. Cumulative biogas production from inocula, Pistia and inoculated Pistia
Number of days 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Biogas production, 1 kg- i (fresh weighO Digester A"
Digester B b
Digester C"
1'10+0'00 1"95+0"05 2"80 + 0"00 3.55+0"05 4'15+0"05 4"70+0"10 5.35+0"05 5'75+0"05 6'25+0"05 6"65+0'05 7-05+0"05 7-35_+0"05 7"80+0"00 8.00+0.00 8'40+0"00
1'70+0'00 2"00+0"00 2"00 + 0.00 2"15+0"15 2.45+0.15 2"45+0"15 2'80+0"20 2'80+0"20 3'15+0"15 3"15+0"15 3-15+0'15 3-30_+0"00 3'45+0"15 3.45 +0"15 3"45-+0"15
7-70+0"10 14"7+0"30 17"95 + 0.65 19"25+0"65 20"20+0"60 20"65+0"65 20-80+0"70 21"25+1.15 21"35+1"25 21'50+1"20 21'90_+1"20 22"15+1.25 22"40+1"10 22"45 + 1"05 22"80+0.70
All values represent the averages of two digesters. "Digester A: Inoculum = 2000 g. bDigester B: Pistia = 300 g; water = 1500 ml. 'Digester C: Pistia = 300 g; inoculum = 1500 g. Temperature: 35°C.
Biogasfrom Pistia
The digesters charged with Pistia alone have significant concentrations of higher fatty acids including propionic, butyric, isobutyric, valeric, and isovaleric acids. In the case of inoculated Pistia propionic acid, only, was detected during the first 4 days of the digestion. Inoculated Pistia has even lower residual acetic acid levels than the inoculum alone. Effect of inocula on a 'soured' digestion Inoculum (1000 ml) was added to the 'soured' digesters (pH 5.0) which had already run on Pistia as feed for 30 days. The gas production on the 30th day, and the cumulative gas production -consisting solely of CO: -- were 0 and 3.5 liters respectively before the addition of inoculum. The concentration of acetic, propionic, butyric, isobutyric, valeric and isovaleric acids at that time
213
were appreciable. Soon after the addition of the inoculum this situation changed dramatically (Table 5). Within 24 h the concentration of acetic acid, propionic, isobutyric, and butyric acids dropped considerably. By the second day the acetic acid concentration had fallen to 323 ppm and that of propionic acid to 11 ppm. The other acids had been consumed to below detectable levels. The gas production also picked up and methane appeared in significant levels (60%) on the first day, building up to 63% by the 7th day. The cumulative gas production per kg VS of Pistia in 25 days was 459 liters compared to 573 liters obtained when Pistia was inoculated at the outset.
Table 5. pH and volatile fatty acids in an inoculated 'soured' digester Initial VFA concentrations: lnocula 221 mg liter- i
Table 3. pH and volatile fatty acids in digesters with inoculum or Pistia + inoculum
Number
pH
Pistia
978 109 113 273 47 31
Volatile fatty acids (rag liter- i)
of days lnoculum Acetic 2 4 6 8 10 15 20 25 30
7-5-7"7 7.5-7.7 7'5-7.6 7.6-7.7 7.5-7.6 7"5-7-7 7"5-8"0 7-6-7-7 7-4-7.5
154_+3 155_+5 131_+1 110+-2 130_+9 91_+5 92+-1 83_+2 102_+6
Pistia + inoculum Acetic
acetic acid (A) propionic acid (P)
3 mg liter J
acetic acid (A) propionic acid (P) isobutyric acid (iB) butyric acid (B) isovaleric acid valeric acid
mg liter I mg liter- ~ mg liter- ~ mg liter- L mg liter- ~ mg liter- ~
Propionic
531_+35 209_+6 90_+8 65+-11 60_+1 44_+5 43+-2 39_+2 63_+3
75+-20 ND ~ ND ND ND ND ND ND ND
Number of days a
lnoculum pH
A
7.8 7.7 7.4 7.4 7.3
71+-3 50+-0 59_+0 39_+3 33_+2
1 2 4 10 25
Inoculated Pistia pH
A
P
7'5 627_+3 7.5 323_+5 7.5 55_+9 7.4 27+-2 7.4 34_+2
51+-2 11_+1 ND ND ND
iB
B
9 ND ND ND ND
ND ND ND ND ND
All values represent the average of two digesters. "After inoculation. ND = Not detectable.
All values represent the averages of two digesters. aND = Not detectable.
Table 4. pH and volatile fatty acids in digesters with P&tia alone
Number of days
pH
Volatile fatty acids (mg liter- 1) Acetic
2 4 6 8 10 12 14 16 20 25 30
4-5-5"3 4.7-5"1 4-9-5"0 4'9-5'0 4'9-5"0 5-9-5"0 4"8-5"0 4"7-4"8 4"8-4"9 4'9-5"0 4'9-5'0
289 + 33 517 + 115 5 6 3 + 100 6 3 0 + 117 734 + 115 782_+ 125 833 + 119 851+_117 939 + 122 904-+94 978_+ 108
Propionic
lsobutyric
3+0 72 ___38 101 + 4 109+7 120 + 8 117+ 11 122 -+ 10 127+-8 142 _+9 94-+ 17 109+-4
ND 5+ 2 8+2 10+2 19 + 5 23+6 34 -+ 10 45_+12 74 +- 14 7 3 + 12 113_+4
All averages represent the averages of two digesters.
Butyric 150 + 34 245 + 1 249+0 267+ 8 282 + 6 282+2 281 _+ 12 297_+8 297 + 12 222_+ 13 273_+37
lsobutyric
Valeric
ND ND 7+7 11 + 6 28 + 1 48+2 61 + 8 42+7 81 + 12 54_+26 47+7
ND ND ND ND 14 + 5 19-+ 1 24_+ 9 31+-14 35 -+ 18 25+9 31 _+7
214
S. A. Abbasi, P. C. Nipaney, M. B. Panholzer
Pistia could, then, be used as a substrate for biogas production provided that it was fed to a working continuous-flow digester or that an inoculum was used for a batch digester. Biogas production could be an incentive to harvest the weed, so reducing pollution of water resources.
ACKNOWLEDGEMENT
The work was financially supported in part by 'Bundesministerium for Auswatige Angelegenheiten', Vienna, Austria and in part by Council of Scientific and Industrial Research, New Delhi; the authors gratefully acknowledge this support.
REFERENCES Abbasi, S. A. & Nipaney, E C. (1990). Bioenergy potential of eight common aquatic weeds. Biol. Wastes, 34, 359-65. Attionu, R. H. (1976). Some effects of water lettuce on its habitat. Hydrobiologia, 50, 245-54. Bua-Ngam, T. & Mercado, B. L. (1975). The life cycle of water lettuce (Pistia stratiotes L.). Philippine Weed Science Bulletin, 2, 11-15. Fachgruppe Wasserchemie (1985). Deutsche Einheitsverfahren zur Wasseruntersuchung. Verlag Chemie, Weinheim, Germany. Rand, M. C., Greenberg, A. R. & Taras, M. J. (1985). Standard Methods for the Examination of Water and Wastewater, 16th edn. APHA, Washington DC. Rao, E N. & Reddy, A. S. (1984). Studies on the population biology of water lettuce: Pistia stratiotes L. Hydrobiologia, l l 9 , 15-19. Tadulingam, E L. S. & Venkatanarayana, G. (1985). A Handbook of Some South Indian Weeds. Periodical Expert Book Agency, New Delhi, pp. 365.
Biogas Production from the Aquatic Weed Pistia
( Pistia stratiotes ) S. A. Abbasi, P. C. Nipaney Salim Ali School of Ecology, Pondicherry (Central) University, Pondicherry 605 001, India
& M. B. Panholzer Institute for Environmental Research, Elisabethstrasse 11, A-8010 Graz, Austria (Received 8 April 1990; revised version received 20 October 1990; accepted 30 October 1990)
Abstract Pistia stratiotes, an aquatic weed, was investigated as a substrate for biogas production in batch digestion. An inoculum was necessary to obtain biogas production from the weed. With Pistia only, production of carbon dioxide alone was high during the first five days of digestion but began to level off thereafter With inoculated Pistia, a high rate of biogas production was sustained for nearly 10 days and the average methane content was 58-68%. The digesters charged with Pistia alone had significant concentrations of propionic, butyric, isobutyric, valeric, and isovaleric acids. These acids were not present in detectable concentrations, in the digesters running with inoculated Pistia, except during the first 4 days of the digestion when propionic acid was formed. When an inoculum was added to a 'soured' digester the performance of the latter improved dramatically.
Key words: Aquatic weed, anaerobic digestion, biogas, batch digestion, Pistia, volatile and fatty acids, methane. INTRODUCTION
Pistia stratiotes L., commonly known as water lettuce or water cabbage, is a pleustonic macrophyte, originally from the states of Florida and Texas, USA (Bua-Ngam & Mercado, 1975; Attionu, 1976; Rao & Reddy, 1984; Tadulingam
& Venkatanarayana, 1985). It is now widespread in the lakes and ponds of the warmer parts of the world. Pistia has a high growth-rate and causes harm to water resources and agriculture in the same manner as other major aquatic weeds. It might be profitably used as a substrate for biogas production.
METHODS Sampling of P/st/a Fresh Pistia (whole) plants were collected from the Botanical Garden, University of Graz, Austria. The plants were washed with water to remove attached sediments, and then finely ground in an 'Ultra Turrax' homogenizer (Janke & Knkel, Staufen, Germany). Source of inoculum Digested slurry from a biogas plant (Harrer Biogas Plant, Graz) running with cow manure at 35°C for several years, was used as an inoculum for the present study. Chemical analysis of P/st/a and inoculum Total solids (TS), volatile solids (VS), nitrogen (N) and pH were determined as the standard methods (Rand et al., 1985). Ammonium nitrogen and chemical oxygen demand (COD) analyses were carried out according to the German Standard Methods of the examination of water, wastewater and sludge (Frachgruppe Wasserchemie, 1985).
211 Bioresource Technology 0960-8524/91/S03.50 © 1991 Elsevier Science Publishers Ltd, England. Primed in Great Britain
S.A. Abbasi, P. C. Nipaney, M. B. Panholzer
212
Volatile fatty acid (VFA) and methane analysis Volatile fatty acids and methane were estimated using a Hewlett Packard gas-liquid chromatograph.
Anaerobic digestion of Pistia All-glass air-tight digesters of 3000 ml capacity with gas sampling and measuring units were employed. Each digester set consisted of a 3000ml round-bottom flask as digester, calibrated gas collecting tube and a bottle to receive displaced water. The digesters had two outlet connections; one for gas and one to collect the slurry for analysis. The gas-collecting tube was filled with acidified water (pH N 2) to prevent absorption of carbon dioxide and hydrogen sulfide gases from the biogas. The digesters were charged with Pistia or Pistia-inoculum mixtures. The controls were with inocula alone.
RESULTS AND DISCUSSION Some characteristics of the feed are presented in Table 1.
Biogas yield The biogas yield per kg (fresh weight) of the feeds over a 30-day retention time is presented in Table 2. The methane content of the biogas from digesters A and C (inoculum/inoculated Pistia) was in the range 52-54% during the first 2 days of the digestion and remained in the range 61-68% for the remaining period. The biogas from digester B (Pistia without inoculum) consisted only of carbon dioxide. As might be expected the presence of inocula has a very pronounced positive effect on the biogas production from Pistia. The pattern of biogas yield -- HRT curves (not shown here) for Pistia was similar to the patterns Table 1. Some characteristics of the feed (wet weight basis)
Parameter
lnocula
Total solids (TS%) Volatile solids (VS%) Chemical oxygen demand (g O2 kg -1) Ammonium nitrogen ( g N k g -~) Kjeldahl nitrogen ( g N k g -~)
6.9 5.4 67
Pistia 4.6 3'9 36
Mixture 5.6 4-4 62
2.2
0.05
1"9
3-7
0-6
3'2
observed with Ceratopteris but different from the ones observed with Salvinia, Azolla, Hydrilla, Nymphaea, Cyperus, Scirpus and Utricularia (Abbasi & Nipaney, 1990). The nature of VS in Pistia is such that the bulk of degradation takes place during the initial phase of the digestion, suggesting that the digesters with Pistia should preferably be run at an HRT close to 10 days for optimal energy yield.
pH The pH of the digesters running on inocula or Pistia-inoculum mixtures was in the mildly alkaline range 7.4-8.0 whereas the digesters with "Pistia alone remained significantly acidic; the pH never rising above 5"3 and often remaining lower than 5 (Tables 3 and 4).
VFA Composition The nature and composition of volatile fatty acids formed during the course of the digestions are shown in Tables 3 and 4. It may be seen that in the case of Pistia, the acetic acid concentrations shoot up to 517 ppm by the 4th day of the digestion and remain in the range 563-978 ppm thereafter, indicating that while the plant is contaminated with fermentative bacteria, methanogenic bacteria are absent from Pistia plants. In contrast inoculated Pistia has a low concentration ( < 70 ppm) of residual acetic acid from the 8th day onwards.
Table 2. Cumulative biogas production from inocula, Pistia and inoculated Pistia
Number of days 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Biogas production, 1 kg- i (fresh weighO Digester A"
Digester B b
Digester C"
1'10+0'00 1"95+0"05 2"80 + 0"00 3.55+0"05 4'15+0"05 4"70+0"10 5.35+0"05 5'75+0"05 6'25+0"05 6"65+0'05 7-05+0"05 7-35_+0"05 7"80+0"00 8.00+0.00 8'40+0"00
1'70+0'00 2"00+0"00 2"00 + 0.00 2"15+0"15 2.45+0.15 2"45+0"15 2'80+0"20 2'80+0"20 3'15+0"15 3"15+0"15 3-15+0'15 3-30_+0"00 3'45+0"15 3.45 +0"15 3"45-+0"15
7-70+0"10 14"7+0"30 17"95 + 0.65 19"25+0"65 20"20+0"60 20"65+0"65 20-80+0"70 21"25+1.15 21"35+1"25 21'50+1"20 21'90_+1"20 22"15+1.25 22"40+1"10 22"45 + 1"05 22"80+0.70
All values represent the averages of two digesters. "Digester A: Inoculum = 2000 g. bDigester B: Pistia = 300 g; water = 1500 ml. 'Digester C: Pistia = 300 g; inoculum = 1500 g. Temperature: 35°C.
Biogasfrom Pistia
The digesters charged with Pistia alone have significant concentrations of higher fatty acids including propionic, butyric, isobutyric, valeric, and isovaleric acids. In the case of inoculated Pistia propionic acid, only, was detected during the first 4 days of the digestion. Inoculated Pistia has even lower residual acetic acid levels than the inoculum alone. Effect of inocula on a 'soured' digestion Inoculum (1000 ml) was added to the 'soured' digesters (pH 5.0) which had already run on Pistia as feed for 30 days. The gas production on the 30th day, and the cumulative gas production -consisting solely of CO: -- were 0 and 3.5 liters respectively before the addition of inoculum. The concentration of acetic, propionic, butyric, isobutyric, valeric and isovaleric acids at that time
213
were appreciable. Soon after the addition of the inoculum this situation changed dramatically (Table 5). Within 24 h the concentration of acetic acid, propionic, isobutyric, and butyric acids dropped considerably. By the second day the acetic acid concentration had fallen to 323 ppm and that of propionic acid to 11 ppm. The other acids had been consumed to below detectable levels. The gas production also picked up and methane appeared in significant levels (60%) on the first day, building up to 63% by the 7th day. The cumulative gas production per kg VS of Pistia in 25 days was 459 liters compared to 573 liters obtained when Pistia was inoculated at the outset.
Table 5. pH and volatile fatty acids in an inoculated 'soured' digester Initial VFA concentrations: lnocula 221 mg liter- i
Table 3. pH and volatile fatty acids in digesters with inoculum or Pistia + inoculum
Number
pH
Pistia
978 109 113 273 47 31
Volatile fatty acids (rag liter- i)
of days lnoculum Acetic 2 4 6 8 10 15 20 25 30
7-5-7"7 7.5-7.7 7'5-7.6 7.6-7.7 7.5-7.6 7"5-7-7 7"5-8"0 7-6-7-7 7-4-7.5
154_+3 155_+5 131_+1 110+-2 130_+9 91_+5 92+-1 83_+2 102_+6
Pistia + inoculum Acetic
acetic acid (A) propionic acid (P)
3 mg liter J
acetic acid (A) propionic acid (P) isobutyric acid (iB) butyric acid (B) isovaleric acid valeric acid
mg liter I mg liter- ~ mg liter- ~ mg liter- L mg liter- ~ mg liter- ~
Propionic
531_+35 209_+6 90_+8 65+-11 60_+1 44_+5 43+-2 39_+2 63_+3
75+-20 ND ~ ND ND ND ND ND ND ND
Number of days a
lnoculum pH
A
7.8 7.7 7.4 7.4 7.3
71+-3 50+-0 59_+0 39_+3 33_+2
1 2 4 10 25
Inoculated Pistia pH
A
P
7'5 627_+3 7.5 323_+5 7.5 55_+9 7.4 27+-2 7.4 34_+2
51+-2 11_+1 ND ND ND
iB
B
9 ND ND ND ND
ND ND ND ND ND
All values represent the average of two digesters. "After inoculation. ND = Not detectable.
All values represent the averages of two digesters. aND = Not detectable.
Table 4. pH and volatile fatty acids in digesters with P&tia alone
Number of days
pH
Volatile fatty acids (mg liter- 1) Acetic
2 4 6 8 10 12 14 16 20 25 30
4-5-5"3 4.7-5"1 4-9-5"0 4'9-5'0 4'9-5"0 5-9-5"0 4"8-5"0 4"7-4"8 4"8-4"9 4'9-5"0 4'9-5'0
289 + 33 517 + 115 5 6 3 + 100 6 3 0 + 117 734 + 115 782_+ 125 833 + 119 851+_117 939 + 122 904-+94 978_+ 108
Propionic
lsobutyric
3+0 72 ___38 101 + 4 109+7 120 + 8 117+ 11 122 -+ 10 127+-8 142 _+9 94-+ 17 109+-4
ND 5+ 2 8+2 10+2 19 + 5 23+6 34 -+ 10 45_+12 74 +- 14 7 3 + 12 113_+4
All averages represent the averages of two digesters.
Butyric 150 + 34 245 + 1 249+0 267+ 8 282 + 6 282+2 281 _+ 12 297_+8 297 + 12 222_+ 13 273_+37
lsobutyric
Valeric
ND ND 7+7 11 + 6 28 + 1 48+2 61 + 8 42+7 81 + 12 54_+26 47+7
ND ND ND ND 14 + 5 19-+ 1 24_+ 9 31+-14 35 -+ 18 25+9 31 _+7
214
S. A. Abbasi, P. C. Nipaney, M. B. Panholzer
Pistia could, then, be used as a substrate for biogas production provided that it was fed to a working continuous-flow digester or that an inoculum was used for a batch digester. Biogas production could be an incentive to harvest the weed, so reducing pollution of water resources.
ACKNOWLEDGEMENT
The work was financially supported in part by 'Bundesministerium for Auswatige Angelegenheiten', Vienna, Austria and in part by Council of Scientific and Industrial Research, New Delhi; the authors gratefully acknowledge this support.
REFERENCES Abbasi, S. A. & Nipaney, E C. (1990). Bioenergy potential of eight common aquatic weeds. Biol. Wastes, 34, 359-65. Attionu, R. H. (1976). Some effects of water lettuce on its habitat. Hydrobiologia, 50, 245-54. Bua-Ngam, T. & Mercado, B. L. (1975). The life cycle of water lettuce (Pistia stratiotes L.). Philippine Weed Science Bulletin, 2, 11-15. Fachgruppe Wasserchemie (1985). Deutsche Einheitsverfahren zur Wasseruntersuchung. Verlag Chemie, Weinheim, Germany. Rand, M. C., Greenberg, A. R. & Taras, M. J. (1985). Standard Methods for the Examination of Water and Wastewater, 16th edn. APHA, Washington DC. Rao, E N. & Reddy, A. S. (1984). Studies on the population biology of water lettuce: Pistia stratiotes L. Hydrobiologia, l l 9 , 15-19. Tadulingam, E L. S. & Venkatanarayana, G. (1985). A Handbook of Some South Indian Weeds. Periodical Expert Book Agency, New Delhi, pp. 365.